Highspeed,
It is true that only 1 of the 3 main AC source (IDG or APU gen) is needed to power the whole a/c electrical need.
As I have said, the ops manuals 2 do not say the BUG is a main AC source. It is only "mentioned" in the non-normal checklist pre-amber as a "main AC source".
Like what 777AV8R said, in any situation, with only 1 of 3 MAIN system remaining, land ASAP. The BUG is exactly what it said it is, a BACK-UP gen, not a main.
Happy flying.